1. Field of the Invention
The present invention relates to a bone cement composition. More particularly, the present invention relates to a bone cement wherein the liquid component comprises a monomer of an acrylic ester and the powdered component comprises a terpolymer of methyl methacrylate, butyl methacrylate, and styrene.
2. Description of the Prior Art
Bone cements find wide usage in a variety of applications. For instance, they are used for cementing implants in place, for the anchoring of endoprostheses of the joints, in the treatment of skull defects, and for the performance of spinal fusion. These cements are typically polymeric materials and the surgeon usually mixes the interactive components to make the cement at an appropriate stage during the surgical procedure. Typically, the components of the bone cement comprise a powdered homopolymer or copolymer of methyl methacrylate and a suitable liquid monomer, for example, methyl methacrylate. To accelerate the polymerization of the bone cement, a catalyst system may also be used. The catalyst, if present, is in the form of a redox catalyst system, usually containing an organic peroxy compound, such as dibenzoyl peroxide, plus a reducing component, such as p-toluidine
Once the bone cement/implant combination, for example, is in the body, the surgeon will later wish to inspect the implant by X-rays and since the polymers and/or monomers are relatively radiolucent, radiopaque materials, also called opacifiers, are added to the polymeric bone cement. Examples of such opacifiers are barium salts, such as barium sulphate, and other salts such as zirconium oxide and zinc oxide. While these opacifying agents give the necessary radiopacity, it has been reported that they tend to reduce the mechanical properties, e.g. transverse strength and compressive strength of the set polymeric bone cement. The reported solution to this alleged problem of reduced mechanical strength is referred to in a number of patents.
U.S. Pat. No. 4,500,658 refers to a method of incorporating an opacifier in an acrylic resin by suspension polymerization.
EPO Patent Application No. 0218471 refers to a composition for forming a bone cement comprising a powdered component and a monomer component, the powdered component comprising ethyl methacrylate polymer beads incorporating particles of opacifier therein and the monomer component comprising n-butyl methacrylate.
U.S. Pat. No. 4,341,691 refers to a low viscosity bone cement comprising a liquid methyl methacrylate monomer and powdered polymethylmethacrylate beads wherein 85-95% of the polymethylmethacrylate beads fall through a #40 mesh and #100 mesh screen and 5-15% of the polymethylmethacrylate beads pass through a #40 mesh screen but not through a #100 mesh screen.
U.S. Pat. No. 4,554,686 refers to a frozen polymethylmethacrylate bone cement.
U.S. Pat. No. 4,268,639 refers to a bone cement prepared by mixing a finely powdered solid polymer phase of polymethylmethacrylate and/or poly (2-hydroxyethyl methacrylate) with a liquid monomer phase of methyl methacrylate and/or 2-hydroxyethyl methacrylate in a weight ratio of polymer phase to monomer phase of 1.5 to 3.3:1.
United Kingdom Patent No. 1,532,318 refers to a bone cement comprising a liquid component comprising methyl methacrylate as an emulsion in water and a powdered component comprising polymethylmethacrylate in finely divided form.
U.S. Pat. No. 4,837,279 refers to a bone cement comprising (a) a liquid component comprising a monomer of an acrylic ester and (b) a powdered component comprising, based on the weight of the powdered component, (i) from 0 to about 20 percent of a methyl methacrylate homopolymer, (ii) from about 30 to about 60 percent of a methyl methacrylatestyrene copolymer, and (iii) from about 30 to about 60 percent of a methyl methacrylate-butyl methacrylate copolymer. Opacifying agents can be incorporated in the powdered component.
The existing bone cement compositions are usually hand mixed at the time of surgery, resulting in materials with a maximum tensile strength of approximately 30 MPa and a maximum tensile deformation of approximately 0.015 strain. With the introduction of new methods of mixing the cement, such as vacuum mixing and centrifugation, improvements in the tensile strength of the bone cement have been reported. While useful for their intended purpose, it would also be highly desirable to have a bone cement composition exhibiting higher maximum tensile deformation, that is, the ability to sustain higher strains without failure.
Since the terpolymer according to the present invention allows the optimum amount of the desired chemical groups in a single polymer chain, it presents a more specific way of varying the mechanical properties of the bone cement.